1. Field of the Invention
The present invention relates generally to a digital signal recording/playback apparatus, and more particularly, to a variable length code recording in the digital signal recording/playback apparatus.
2. Description of Related Art
A digital processing of video data has been developed in recent years. In particular, various systems for recording digital video data using a recording medium such as magnetic video cassette recorders (VCRs) have been developed. FIGS. 1(a) and 1(b) illustrate diagrams for the relations of the positions on a screen and the positions on the recording tracks of a recording medium in VCRs. FIG. 1(a) indicates the positions on the screen and FIG. 1(b) indicates the positions on the recording tracks.
FIG. 1(a) illustrates one frame of picture vertically divided into eight sections. FIG. 1(b) indicates the record positions of the first through ninth tracks similarly divided into eight tracks. Video data is recorded in order on a recording medium starting from the lowest line position A of the first track to its top line position I. For instance, when recording one frame of data on one track, data displayed in a horizontal section defined by lines a and b, FIG. 1(a), on a screen are recorded on a longitudinal section defined by lines A and B on a recording medium, and thereafter, in the similar manner data displayed in horizontal sections defined by lines b through i on the screen are recorded in the order on longitudinal sections defined by lines B through I on the recording medium. When recording one frame of data on two tracks, data in the horizontal section defined by the lines a and e on the screen are recorded on the longitudinal section defined by the lines A and I of the first track #1, while data in the horizontal section defined by the lines e and i on the screen are recorded on the longitudinal section defined by the lines E and I of the second track #2.
FIGS. 2(a) through 2(d) are diagrams for illustrating the relationship between trace patterns and playback signal envelopes of recorded data at triple speed. FIG. 2(a) illustrates trace patterns when playback data at triple speed with a head scanning time shown at the abscissa axis and track pitch or tape traveling distance at the coordinate axis. The signs "+" and "-" in the diagram indicate differentially oriented normal azimuths of the playback head, respectively. Further, numerals in the diagram show track numbers; thus odd number tracks are in the plus azimuth and even number tracks are in the minus azimuth. FIGS. 2(b) through 2(d) indicate the signal envelope played back by the ordinary head, the playback envelope by the special purpose head and the synthetic playback envelope obtained by both heads.
FIG. 3 is a diagram for illustrating the construction of the recording/playback heads. Assume that a rotary cylinder 3, as shown in FIG. 3, is used for the data recording and playback operations. The rotary cylinder 3 is equipped with a pair of regular heads 1, which have differentiated azimuths with each other. Rotary cylinder 3 is also equipped with a pair of special purpose heads 2, which have differentiated azimuths with each other. Furthermore, the azimuths of one regular head 1 and its adjacent special purpose head 2 are differentiated with each other. As shown by the sign "+" in FIG. 2(a), the first track and the third track are traced by the regular head 1 of the plus azimuth in the initial scanning period (tracing period), and the fourth track and the sixth track are traced by the regular head 1 of the minus azimuth in the next scanning period. Thus, the playback signal envelope shown in FIG. 2(b) is obtained by the regular head 1. Further, the second track is traced by the special purpose head 2 in the initial scanning period and the playback signal envelope shown in FIG. 2(c) is obtained in the same manner. By combining the playback output from the regular head 1 with the playback output from the special purpose head 2, synthetic playback envelope shown in FIG. 2(d) obtained.
Table 1 shown below indicates relations among the playback outputs at the triple speed playback operation (FIG. 2(d)), the traces of head and the corresponding positions on the screen.
TABLE 1 ______________________________________ Playback 1 Frame/1 Track 1 Frame/2 Tracks Track Track Frame Track Frame ______________________________________ 1 #1 1st Frame #1 1st Frame (A)-(C) (a)-(c) (A)-(C) (a)-(c) 2 #2 2nd Frame #2 1st Frame (C)-(G) (c)-(g) (C)-(G) (f)-(h) 3 #3 3rd Frame #3 2nd Frame (G)-(I) (g)-(i) (G)-(I) (d)-(e) 4 #4 4th Frame #4 2nd Frame (A)-(C) (a)-(c) (A)-(C) (e)-(f) 5 #5 5th Frame #5 3rd Frame (C)-(G) (c)-(g) (C)-(G) (b)-(d) 6 #6 6th Frame #6 3rd Frame (G)-(I) (g)-(i) (G)-(I) (h)-(j) 7 #7 7th Frame #7 4th Frame (A)-(C) (a)-(c) (A)-(C) (a)-(b) 8 #8 8th Frame #8 4th Frame (C)-(G) (c)-(g) (C)-(G) (f)-(h) 9 #9 9th Frame #9 5th Frame (G)-(I) (g)-(i) (G)-(I) (a)-(b) ______________________________________
As shown in FIG. 2(d) and Table 1, data A through C on the first track #1 are played back by the regular head 1 in the first quarter (1/4) time interval in the initial scanning period, data of C through G on the second track #2 are played back by the special purpose head 2 in the next half (1/2) time interval, and data of G through I on the third track are played back by the regular head 1 in the next 1/4 time interval. Thereafter, data on three tracks are played back in the similar manner in one scanning period.
When one video data frame is recorded on one track, the positions of A through C on the first track #1 correspond to the positions a through c on the first frame of image, the positions C through G on the second track #2 correspond to the positions c through g on the second frame of the frame, and the positions G through 1 on the third track #3 correspond to the positions g through i on the third frame of the image, as shown in Table 1. Therefore, in the playback operation at triple speed, the picture patterns at the positions on the first through the third frames are combined and displayed as a playback picture.
When one video data frame is recorded on two tracks, the positions A through C on the first track #1 correspond to the positions a and b on the first frame, the positions C through G on the second track #2 correspond to the positions f through h on the first frame, and the positions G through I on the third track #3 correspond to the positions d through e on the second frame as shown in Table 1. Further, the positions A through C on the fourth track #4 correspond to the positions e and f on the second frame, the positions C through G on the fifth track #5 correspond to the positions b through d on the third frame, and the positions G through I on the sixth track #6 correspond to the positions h through i on the third frame. In this case, therefore, the picture patterns at the positions on the first through the third frames are presented in mix on the playback picture as shown in FIG. 4(b).
Various proposals have been proposed in recent years for the standardization of high efficient encoding for compressing video data.
The high efficient encoding technique encodes video data at a lower bit rate in order to improve efficiency of digital transmission and recording. For instance, the CCITT (Cometi Consultafif international Telegraphique et Telephonique (International Telegraph and Telephone Consultative Committee)) has issued a recommendation for video-conference/video-telephone standardization H. 261. According to the CCITT recommendation, the encoding of frame I is processed by intra-frame compression and frame P is processed by inter-frame compression (or a predictive frame compression).
FIG. 5 is a diagram for explaining video data compression according to the CCITT recommendation.
Frame I is processed by intra-frame compression and is the only video data frame encoded by DCT (Digital Cosine Transformation) processing. The inter-frame compression processed frame P is video data encoded by a predictive encoding method using the intra-frame compression processed frame I or the inter-frame compression processed frame P. In addition, lowering of bit rate has been achieved by encoding the data in variable lengths. As the intra-frame compression processed frame I was encoded by the intra-frame information only, it is possible to decode it using singly encoded data. However, the inter-frame compression processed frame P was encoded using the correlation with other video data, it cannot be decoded by using only singly encoded data.
FIG. 6 is a block diagram for illustrating the recording section of a conventional apparatus for recording/playback variable length codes using predictive encoding.
A luminance signal Y and color difference signals Cr and Cb are applied to a multiplexer 11, where they are multiplexed in a block of 8 pixels.times.8 horizontal scanning lines. Sampling rate of the color difference signals Cr and Cb in the horizontal direction is a half (1/2) of the luminance signal Y. Therefore, in the period when two 8.times.8 luminance blocks are sampled, one 8.times.8 block of the color difference signals Cr and Cb is sampled. As shown in FIGS. 7(a) through 7(c), two luminance signal blocks Y1 and Y2 and each of the color difference signal blocks Cr and Cb total four blocks called a macro block. Here, two luminance signal blocks Y1 and Y2 and each of the color difference blocks Cr and Cb represent the same position of the picture frame. The output of the multiplexer 11 is applied to a DCT (Digital Cosine Transformation) circuit 13 through a subtracter 12.
When performing intra-frame compression, a switch 14 is kept OFF and the output of the multiplexer 11 is applied directly to the DCT circuit 13, described later. A signal composed of 8.times.8 pixels per block is applied to the DCT circuit 13. The DCT circuit 13 converts the input signal into frequency components by the 8.times.8 two dimensional DCT processing. This makes it possible to reduce the spatial correlative components. That is, the output of the DCT circuit 13 is applied to a quantizer 15 which lowers one block signal redundancy by requantizing the DCT output using a fixed quantization coefficient. Further, block pulses are supplied to the multiplexer 11, the DCT circuit 13, the quantizer 15, etc. which operate in a block unit.
The quantized data from the quantizer 15 is applied to a variable length encoder 16 and is, for instance encoded to the Huffman codes based on the result calculated from the statistical encoding amount of the quantized output. As a result, a short bit is assigned to data having a high appearance probability and a long bit to data having a low appearance probability and thus, transmission amount is further reduced. The output of the variable length encoder 16 is applied to an error correction encoder 17, which provides the output from the variable length encoder 16 with an error correction parity added to a multiplexer 19.
The output of the variable length encoder 16 is also applied to an encoding controller 18. The amount of the output data varies largely depending on input picture. So, the encoding controller 18 monitors the amount of the output data from the variable length encoder 16 and regulates the amount of the output data by controlling the quantization coefficient of the quantizer 15. Further, the encoding controller 18 may restrict the amount of the output data by controlling the variable length encoder 16.
A sync/ID generator 20 generates frame a sync signal and ID signal showing data contents and additional information and provides them to the multiplexer 19. The multiplexer 19 forms one sync block data with a sync signal, ID signal, compressed signal data and parity. The multiplexer 19 provides this data to the recoding/encoder which is not shown. The recording/encoder, after recording/encoding the output from the multiplexer 19 according to characteristic of a recording medium, records the encoded data on a recording medium (not shown).
If the switch 14 is ON, the current frame signal from the multiplexer 11 is subtracted from the motion compensated preceding frame data (which will be described later), in the subtracter 12 and applied to the DCT circuit 13. That is, in this case, inter-frame encoding is carried out to encode differential data using redundancy of inter-frame picture. When a difference between the preceding frame and the current frame is obtained by inter-frame encoding, it will become large if there is any motion in the picture. The differential value is made small by compensating the motion. Compensation is achieved by obtaining a difference at the pixel position corresponding to the motion vector while detecting the motion vector by obtaining the position of the preceding frame corresponding to the fixed position of the current frame. That is, the output of the quantizer 15 is also applied to an inverse quantizer 21. This quantized output is inverse-quantized in the inverse quantizer 21 and inverse DCT processed in an inverse DCT circuit 22 and restored to the original video signal. Further, the original information cannot be played back completely in the DCT processing, requantization, inverse quantization and inverse DCT processing because part of the information is missing. In this case, as the output of the subtracter is a differential information, the output of the inverse DCT circuit 22 is also a differential information. The output of the inverse DCT circuit 22 is applied to an adder 23. This output from the adder 23 is fed back through a variable delay circuit 24 which delays signal by about one frame period and a motion compensator 25, and the adder 23 restores the current frame data by adding differential data to the preceding frame data and provides them to the variable delay circuit 24.
The preceding frame data from the variable delay circuit 24 and the current frame data from the multiplexer 11 are applied to a motion detector 26 where a motion vector is detected. The motion detector 26 obtains a motion vector through a full search motion detection by, for instance, a matching calculation. In the full search motion detection, the current frame is divided into a fixed number of blocks and the search range of, for instance, 15 horizontal pixels.times.8 vertical pixels are set for each block. In the search range corresponding to a preceding frame, the matching calculation is carried out for each block and an inter-pattern approximation is calculated. Then, by calculating the preceding frame block which provides the minimum distortion in the search range, the vector which is obtained by this block and the current frame block is detected as the motion vector. The motion detector 26 provides the motion vector to the motion compensator 25.
The motion compensator 25 extracts corresponding block data from the variable delay circuit 24, corrects it according to the motion vector and provides it to the subtracter 12 through the switch 14 and also, to the adder 23 after making the time adjustment. Thus, the motion compensated preceding frame data is supplied from the motion compensator 25 to the subtracter 12 through the switch 14. Thus, when the switch 14 is ON, inter-frame compression mode results and if the switch 14 is OFF, intra-frame compression mode results.
The switch 14 is turned ON/OFF based on a motion judging signal. That is, the motion detector 26 generates the motion judging signal depending on whether the motion vector size is in excess of a fixed threshold value and outputs it to a logic circuit 27. The logic circuit 27 controls the ON/OFF of switch 17 by logical judgment using the motion judging signal and a refresh periodic signal.
The refresh periodic signal is a signal showing the intra-frame compression processed frame I illustrated in FIG. 5. If the input of the intra-frame compression processed frame I is indicated by the refresh periodic signal, the logic circuit 27 turns switch 14 OFF irrespective of the motion judging signal. If the motion judging signal indicates that the motion is relatively fast and the distortion calculated by the matching calculation exceeds a minimum threshold value, the logic circuit 27 turns the switch 14 OFF and the intra-frame encoding is carried out for each block even when the inter-frame compression processed frame P data are input. Table 2, shown below, indicates the ON/OFF control of switch 14 by logic circuit 27.
TABLE 2 ______________________________________ Frame I Intra-Frame Compression Switch 14 OFF Processed Frame Frame P Motion Vector Detected Switch 14 ON Inter-Frame Compression Processed Frame Motion Vector Unknown Switch 14 OFF Inter-Frame Compression Processed Frame ______________________________________
FIG. 8 is a diagram for illustrating the data stream of record signals which are output from the multiplexer 19.
As shown in FIG. 8, the first and the sixth frames of the input video signal are converted to the intra-frames I1 and I6, respectively and the second through the fifth frames are converted to the inter-frame compression processed frames P1 through P5. The ratio of data amount between the intra-frame compression processed frame I and the inter-frame compression processed frame P is (3-10):1. The amount of data of the intra-frame compression processed frame I is relatively large, while the amount of data of the inter-frame compression processed frame P is extremely reduced. Further, the data of the inter-frame compression processed frame P cannot be decoded unless other frame data is decoded.
FIG. 9 is a block diagram indicating the decoding section (playback section) of a conventional variable length code recorder.
Compressed, encoded data recorded on a recording medium is played back by the playback head which is not shown and applied into an error correction decoder 31. The error correction decoder 31 corrects errors produced in data transmission and recording. The playback data from the error correction decoder 31 are applied to a variable length data decoder 33 through a code buffer memory 32 and decoded to fixed length data. Further, the code buffer memory 32 may be omitted.
The output from the variable length decoder 33 is inverse quantized in an inverse quantizer 34, inverse DCT processed and decoded to the original video signal in an inverse DCT circuit 35, and applied to a terminal of switch 36. The output of the variable length decoder 33 is also applied to a header signal extractor 37. The header signal extractor 37 retrieves a header showing whether the input data is the intra-frame compression data or the inter-frame compression data and provides it to switch 36. When supplied with a header showing intra-frame compression data, switch 36 selects terminal a and outputs decoded data from the inverse DCT circuit 35.
The inter-frame compression data is obtained by adding the output from the inverse DCT circuit 35 and the preceding frame output from the predictive decoder 39 using an adder 38. That is, the output of the variable length decoder 33 is applied to a motion vector extractor 40 and the motion vector is thus obtained. This motion vector is applied to a predictive decoder 39. The decoded output from the switch 36 is delayed for one frame period by a frame memory 41. The predictive decoder 39 compensates the preceding frame decoded data from the frame memory 41 according to the motion vector and provides them to the adder 38. The adder 38 decodes the inter-frame compression data by adding the output from the predictive decoder 39 and the output from the inverse DCT circuit 35 and provides the decoded inter-frame compression data to the terminal b of the switch 36. When the inter-frame compression data is applied, switch 36 selects terminal b based on the header and outputs the decoded data from the adder 38. Thus, the compression and expansion are carried out without delay.
However, intra-frame compression processed frame I and inter-frame compression processed frame P differ in encoded amounts. Thus, if the data stream shown in FIG. 8 is recorded on a recording medium, one frame will not necessarily be able to be played back in a playback operation at a triple speed. Further, the inter-frame compression processed frame P processed inter-frame compression will become unable to playback if any undecoded frame is generated in the playback operation at triple speed because it cannot be decoded for an independent frame.
Thus, the conventional variable length code recorder described above has a problem in that the picture quality played back in the special playback operation deteriorates greatly because each data frame is of variable length and some data frames of a single frame cannot be decoded.
Since data of every frame is in variable length and since there is frame data which cannot be decoded as described above, picture quality deteriorates in special playback operation. However, it is possible on a VCR having a high efficiency encoding process to play back encoded data to some extent while suppressing error propagation by synchronizing the signal, etc. However, if there is data which is not transmitted in circumstances, such as video-conference, video-telephone, etc., where high efficiency encoded signals are applied to, for instance, a TV set, errors are extensively propagated and the quality of displayed images is deteriorated.
This problem will be discussed in reference to FIG. 10, FIG. 11 and FIGS. 12(a) through 12(f). FIG. 10 is a diagram for explaining a broadcasting system adopted for video-conference, video-telephone, etc.
In a broadcasting station, video signals from cameras (not shown) are high-efficiency encoded in an encoder 151 and added with an error correction code in an error correction encoder 152 corresponding to a transmission line 154. A transmission modulator 153 modulates the output from the error correction encoder 152 corresponding to the transmission line 154 prior to placing the output on the transmission line 154. At the receiving section, signals received through the transmission line 154 are demodulated in a receiving demodulator 155. An error corrector 156 corrects errors generated in the transmission line 154 and feeds them to a switch 157 and also to a VCR 158. The VCR 158 records the input signals or plays back the signals and feeds them to the switch 157. The switch 157 is switched by an input switching signal based on user operation. The switch 157 selects either the output of the error corrector 156 or that of the VCR 158 and provides it to a decoder 159. The decoder 159 decodes the high-efficiency encoded signals back to original signals and an error corrector 160 corrects errors remaining in the decoded output and provides them to a monitor TV set (not shown). Thus, the broadcasting signals applied through the transmission line 154 or playback signals from the VCR 158 are displayed on the screen of the monitor TV set.
FIG. 11 is a block diagram for illustrating the construction of a VCR which is capable of high-efficiency encoding and decoding. Further, the VCR shown in FIG. 10 is of the same construction as shown to the right of the broken line in FIG. 11.
Video signals are high-efficiency encoded in an encoder 161 and then applied to an error correction encoder 162. The error correction encoder 162 provides encoded data with an error correction parity code adapted to a VCR added at adder 163. The adder 163 adds a synchronizing signal and ID signal, generated in an ID generator 164, to the output of the error correction encoder 162 and provides the output to a recording/modulator 165. The recording/modulator 165 modulates this output and provides it to a recording amplifier 166. This recording amplifier 166 amplifies the modulated signals and feeds them to a magnetic head 167 for recording on a tape 168.
In a playback operation of recorded signals, the tape 168 is traced by the magnetic head 167 to play back recorded signals and the playback signals are supplied to a playback amplifier 169. The playback signals from the playback amplifier 169 are waveform equalized in a waveform equalizer 170 to reduce inter-code interference and then applied to a synchronizer 171. The synchronizer 171 restores the playback data in units of recorded data and feeds them to a demodulator 172. The demodulator 172 demodulates the playback data and feeds them to an error corrector 173. The error corrector 173 corrects errors in the playback data and provides to a decoder 174. The decoder 174 and an error corrector 175, identical to the decoder 159 and error corrector 160 shown in FIG. 10, decodes the output of the error corrector 173 and after correcting errors, outputs the error corrected output.
It is assumed that the switch 157 shown in FIG. 10 selects the VCR 158. Data transmitted from a broadcasting station via the transmission line 154 are supplied to the VCR 158. Data transmitted from a broadcasting station via the transmission line 154 are supplied to the error corrector 156. Thus, the recording data train shown in FIG. 12(a) is applied to the VCR 158. In FIG. 12(a), the subscript n denotes track number and the subscript m denotes recorded data train number. That is, Gn, m denote the mth data train on the nth track.
If the data train Gn,1 through Gn, m is recorded in the VCR 158 and this data train is played back without error, the reencoded data train will become identical to the playback data train in the normal playback operation as shown in FIG. 12(b). However, data is played back by the magnetic head while crossing the tracks in the playback operation at triple speed, as described above, and therefore, the playback data does not agree with the recorded data. That is, as shown in FIG. 12(c), the k0 data train through the k1 data train are played back on the first track, the k2 through k3 data trains are played back on the second track, and the k4 through k5 data trains are played back on the third track.
The VCR 158 carries out the demodulation, error correction and decoding processes of this playback data. However, data may not be played back correctly at portions where the recording tracks are switched. Furthermore, the playback data trains become discontinuous at switching points of the recording tracks. Thus, data around the track switching points cannot be used for decoding. Further, in the VCR 158, video data is recorded with a synchronizing signal and ID signal added and demodulated in the synchronizer 171 when played back. Therefore, if data is not played back in the middle of a synchronizing block, it is possible to demodulate data from the starting position of the next synchronizing block. Thus, the portion shown by the broken lines in FIG. 12(d) are not output against FIG. 12(c).
However, if header and address are added to data, a monitor TV set is not able to reconstruct images using the data and simply displays images in order of input image data. Transmitted data trains are of variable length. Even when data length of the broken lined part shown in FIG. 12(d) is known, it is not possible to identify the start position of next data train k2's. Accordingly, it is not possible to display playback images using all playback outputs from the VCR 158 on the screen of a monitor TV set because information data is not used effectively when an error flag is added to it. That is, in such a system as video-telephone, which decodes input data trains continuously, if data is interrupted, subsequent data cannot be used efficiently.
So, in order to stop error propagation, decoding may be enabled for a fixed period of the top of a track by specifying the top of the track as the start position of a data train as shown in FIG. 12(e). This will make it possible to decode a data train G1, K0' through G1, K1'. Further, FIG. 12(f) illustrates an example where the data position with a .DELTA. mark was set as the data train start position. In this case, the data train G1, 11 through G1, 12 can be decoded.
As described above, a problem occurs if discontinuous data is transmitted because data available for effective use in playback operation of images decreases.
A playback signal processing method which is capable of special playback operation using the above special playback heads is shown in FIG. 13. In this drawing, a signal played back by a normal head 210 is amplified in an amplifier 211 and supplied to a detector 250 and a switch 223. Similarly, a signal played back by a special purpose head 216 is amplified in an amplifier 217 and supplied to a detector 251 and switch 223. In the detectors 250 and 251, envelop constituents of respective signals are detected and provided to an amplitude comparator 252. In the amplitude comparator 252, amplitudes of these two signals are compared and the result is supplied to a switch 253.
A switching signal is also supplied to the switch 253 and one of these signals is selected by a mode selecting signal and supplied to the switch 223 as a control signal. The switch 223 selects one of the outputs from the amplifiers 211 and 217 according to this control signal and supplies it to a demodulator 224 where the output signal is demodulated. This demodulated signal is restored to the original image signal in a playback processor 254.
The switching signal described above is used to select a preset head output signal. If this signal is selected by the switch 253, the outputs from the amplifiers 211 and 217 are selected alternately. In the amplitude comparator 252, a signal of larger amplitude is normally selected. Therefore, the output of the amplitude comparator 252 is selected by the mode selecting signal in the switch 253 and the output of larger amplitude from the amplifiers 211 and 217 is selected at the switch 223.
FIG. 14 illustrates trace patterns in the playback operation at triple speed. In this drawing, although the tracks are aslant on a tape, the trace of the head is vertical to the traveling direction of a tape because of triple speed. Therefore, the trace of the head extends over the tracks b, c and d. However, because the azimuth angles of the tracks b and c differ from those of the tracks c and d, data on Trace 1 is played back by selecting data played back, for instance, by the playback heads from S to A out of Trace 1, by the special purpose heads from A to B and by the playback heads from B to E.
If analog signals are played back in this way, an image without a noise bar is obtainable on a screen. However, as data is recorded on a tape in block units when digitally processed, the signals may be switched in the middle of a block if signals played back from two heads are switched according to the amplitude or at a preset switching point. In this case, because data of a block makes no sense, invalid data is produced before or after the switching. This state is shown in FIGS. 15(a) and 15(b).
FIG. 15(a) indicates data played back by the playback head and FIG. 15(b) is data played back by the special purpose head. "SY", "ID", "D" and "PA" indicate synchronizing signals, ID signals, data and parity signals, respectively. "SY" through "PA" form one block.
Here, assuming that the switching points are set at the points A and B, signals are switched in the middle of data D. If the switching point is A, signals are synchronized as the positions of both "SY" are in accord with each other, but data is invalid because the tracks are different. So, the portion of A in FIG. 15(a) and the portion of B in FIG. 15(b) become invalid data. If the switching point is B, signals are not synchronized as both of the two "SY"'s are out of position. As described above, if the switching point is preset or set according to the amplitude, invalid data is produced before and after the switching point and in addition, signals can not be synchronized in some cases.
When a recorded video tape is played back in a VCR at triple speed, the trace of the head crosses three tracks on a magnetic tape. To prevent azimuth angles from becoming different or noise bars from being generated on a screen, special playback heads are provided adjacent to ordinary playback heads and recorded data is played back by switching signals played back by the respective playback heads. However, when recorded data is digitally processed, invalid data is produced before or after the switching point if the switching point is preset or set according to signal amplitude. Furthermore, signals can not be synchronized in some cases.